The present invention concerns heart-monitoring devices and methods, particularly implantable defibrillators, pacemakers, and cardioverters, and methods for processing heart-signal data.
Since the early 1980s, thousands of patients prone to irregular and sometimes life threatening heart rhythms have had miniature heart-monitoring devices, such as defibrillators, pacemakers, and cardioverters, implanted in their bodies. These devices detect onset of abnormal heart rhythms and automatically apply corrective electrical therapy, specifically one or more bursts of electric current, to their hearts. When the bursts of electric current are properly sized and timed, they restore normal heart function without human intervention, sparing patients considerable discomfort and often saving their lives.
The typical implantable heart-monitoring device includes a set of electrical leads, which extend from a sealed housing through the veinous system into the inner walls of a heart after implantation. Within the housing are a battery for supplying power, a capacitor for delivering bursts of electric current through the leads to the heart, and heart-monitoring circuitry for monitoring the heart and determining not only when and where to apply the current bursts but also their number and magnitude. The monitoring circuitry generally includes a microprocessor and a memory that stores instructions directing the microprocessor to interpret electrical signals that naturally occur in the heart as normal or abnormal rhythms. For abnormal rhythms, the instructions, or more generally signal-processing algorithm, also tell the processor what, if any, electrical therapy should be given to restore normal heart function.
In general, these algorithms use the time intervals between successive heart beats, or cardiac events, as a key determinant of therapy decisions. Thus, mistakes in determining these intervals can ultimately undermine the validity of resultant therapy decisions.
Determining these intervals can be especially problematic in dual-chamber devices, which monitor the beats of two chambers of the heart, the left ventricle and the left atrium. In these devices, there is a significant risk of mistaking a ventricle beat for an atrial beat, and therefore counting too many atrial beats and miscalculating some atrial intervals (the time between atrial beats). Because of this risk, many dual-chamber devices include safeguards to ensure accuracy of atrial interval measurements.
There are three basic approaches to designing these safeguards. The first approach, called cross-chamber blanking, entails using a blanking period to prevent sensing atrial beats within a certain time period after the last ventricular beat. In other words, atrial sensing is temporarily disabled after each ventricular beat to prevent mistaking the ventricular beat for an atrial beat. Blanking, however, forces some devices to overlook any atrial beats that might occur during the blanking period. The second approach, which is corrective rather than preventative, entails looking for certain patterns in atrial and ventricular electrograms (signal charts) to identify a ventricular beat mistaken for an atrial beat. If a mistake is detected, this approach discards a select portion of the atrial electrogram. (See U.S. Pat. No. 5,759,196.) Unfortunately, discarding a portion of the atrial electrogram delays the making of therapy decisions. U.S. Pat. No. 5,755,739 reports a third approach which adaptively filters out parts of an atrial electrogram and uses morphologic techniques to verify accuracy of the filtered atrial electrogram. Unfortunately, this corrective approach requires extensive computations and thus not only delays therapy decisions, but also consumes considerable battery power. Accordingly, there is a need for other methods of ensuring accurate interval measurements.
To address this and other needs, the inventor has devised new methods for processing heart electrical signals, some of which ensure accurate interval measurements without unduly delaying therapy decisions or consuming significant battery power. One of these new methods identifies, or detects, an abnormal interval measurement and then either disqualifies the abnormal interval from use in making therapy decisions or divides the abnormal interval into two or more other intervals.
More particularly, an exemplary embodiment, or implementation, of this method entails first identifying an abnormal interval which has a predetermined size relative one or more preceding intervals, with each interval representing a time between successive atrial events. The method then determines whether a ventricular event occurred during a specific portion of the abnormal interval. If a ventricular event occurred, the method either disqualifies the abnormal interval from further processing or divides it into two or more new intervals for use in further processing, such as computing an average atrial interval.
Ultimately, this and other methods embodying teachings of the present invention, can be incorporated into medical devices, for example, implantable pacemakers, defibrillators, or cardioverter defibrillators, to identify and treat abnormal rhythmic conditions both efficiently and accurately.